In recent years, the miniaturization of unit pixel cells in a charge coupled device (CCD) or metal oxide semiconductor (MOS) solid-state imaging device (image sensor) has been rapidly advanced. The solid-state imaging device has a photodiode (photoelectric conversion unit) in a semiconductor substrate made of crystalline silicon, and a CCD or a MOS as a scanning circuit. The cell size (size of unit pixel cell) was 3 μm around 2000, decreasing to 2 μm or less around 2007. Solid-state imaging devices having unit pixel cells of around 1.4 μm are expected to be put into production in 2010. If the miniaturization of cells is being advanced at this pace, the size of the cells is expected to be less than 1 μm within the next several years.
However, there are two major problems to realize a cell size of 1 μm or less. A first problem is due to the small optical absorption coefficient of crystalline silicon. A second problem relates to a signal amount to be handled.
The following details the first problem. The optical absorption coefficient of crystalline silicon depends on the wavelength of light. Crystalline silicon having a thickness of 3.5 μm is necessary for absorbing almost 100% of green light near a wavelength of 550 nm which decides the sensitivity of an image sensor and photo-electrically converting. Therefore, it is necessary to photo-electrically convert and collect signal charges on the condition that the depth of a photodiode to be formed in a semiconductor substrate is around 3.5 μm. However, it is extremely difficult to form a photodiode having a planar cell size of 1μ square and a depth of around 3.5 μm. Even if a photodiode having a depth of around 3.5 μm can be formed, a problem may arise with high possibility that obliquely incident light is incident on the photodiodes of adjacent pixel cells. When the obliquely incident light is incident on the photodiodes of the adjacent pixel cells, color mixture (crosstalk) is caused. This is a major problem for color solid-state imaging devices. Meanwhile, when a photodiode having a depth less than 3.5 μm is formed to prevent the color mixture, the absorption efficiency of green light and the sensitivity of an image sensor are degraded. In the miniaturization of a unit pixel cell, the sensitivity of a unit pixel cell is degraded with decrease in a cell size. Therefore, degradation in light absorption efficiency in addition to this is fatal. The issue of the color mixture arises in varying degrees. For example, when a depth of 3.5 μm is required, the issue of the color mixture arises at a size of a unit pixel cell of around 3.5μ square or less.
The following describes the second problem. The signal amount to be handled depends on the saturation charge amount of a buried photodiode having a photodiode structure used in almost all the crystalline-silicon image sensors. The buried photodiode has the advantage that it can transfer almost 100% of signal charges stored therein to an adjacent charge detection unit (complete transfer). Therefore, as little noise relating to charge transfer is generated, this buried photodiode is widely used for image sensors. However, the capacity of the buried photodiode per unit area cannot be increased in order to achieve this complete transfer. Therefore, the saturation charge amount is needed to be decreases with the miniaturization of a unit pixel cell. In a compact digital camera, 10,000 saturation electrons per unit pixel cell are required. However, when the size of the unit pixel cell is around 1.4 μm, the limit is around 5000 electrons. To response to decrease in the number of saturation electrons, an image is generated by, for example, noise suppression processing performed by the technique of digital signal processing. However, it is difficult to obtain a natural reproduced image. Furthermore, for high-quality single lens reflex cameras, it is said that around 30,000 saturation electrons are necessary per unit pixel cell.
It should be noted that by abrading the surface of the crystalline silicon substrate of a MOS image sensor, the structure in which light is incident not on the front side where a pixel circuit is formed, but on the backside is brought into consideration. This structure is considered effective in the miniaturization of a unit pixel cell. Although this structure has the advantage that incident light is less likely to be prevented by lines and others making up the pixel circuit. However, even with this structure, the two problems cannot be solved at all.
The stacked image sensors disclosed in Patent Literatures 1, 2, and 3 are examples of the structure to solve the two problems. The stacked image sensors have the structure in which a photoelectric conversion film is formed, via an insulating film, above a semiconductor substrate (crystalline silicon) where a pixel circuit is formed. This enables the selection of a material having a large optical absorption coefficient for the photoelectric conversion film. Thus, the first problem can be solved. For instance, when amorphous silicon is used for the photoelectric conversion film, green light having a wavelength of 550 nm can be mostly absorbed at a film thickness of 0.4 μm. That is, green light can be absorbed at a thickness which is around single-digit less than that of crystalline silicon. It is rare that the absorption coefficient of crystalline silicon is small. This is because the transition of electrons in the band gap of a semiconductor is indirect transition. Moreover, since a buried photodiode is not used, it is possible to use a large capacity for a photoelectric conversion unit, and increase saturation electrons. Furthermore, since the charges are not completely transferred, the addition of capacitance can be facilitated, and sufficiently large capacitance of the photoelectric conversion unit can be achieved even in a miniaturized unit pixel cell. Thus, the second problem can be also solved. The image sensor may have a structure as the stack cell of a dynamic random access memory (DRAM).